1. Field of the Invention
The invention is related to the field of disk drive systems, and in particular, to disk drive systems and circuitry that bias the error signal for the adaptive filter in the read channel.
2. Statement of the Problem
FIG. 1 depicts a conventional system that includes a host computer system 100 and a disk drive system 102. The disk drive system 102 includes control circuitry 104 and disk device 106. The disk device 106 stores data for the computer system 100. To transfer this data from the disk device 106 to the computer system 100, the disk device 106 transfers a signal 120 to the control circuitry 104. The signal 120 is an analog representation of the data. The control circuitry 104 converts the signal 120 into a signal 126 for the computer system 100. The signal 126 is a digital representation of the data and is suitable for processing by the computer system 100. Thus, the control circuitry 104 converts an analog representation of the data into a digital representation of the data.
Those skilled in the art will appreciate that numerous conventional components of the disk drive system 102 are not depicted on FIG. 1 for the purpose of clarity. For example, the disk device 106 typically includes disks on which data is written, heads to write/read the data to/from the disks, and motors that position heads and rotate the disks. The control circuitry 104 typically includes a controller, servo circuitry, and a read channel. The controller manages data transfers. The servo circuitry controls the motors to position the heads and rotate the disks. The read channel converts the analog signals from the disks into usable digital data. The read channel includes an adaptive filter 108 and an error signal circuit 110 that are shown on FIG. 1.
The adaptive filter 108 is a digital Finite Impulse Response (FIR) filter that receives a signal 121. The signal 121 is a sampled version of the signal 120. The adaptive filter 108 processes the samples in the signal 121 to generate the signal 122. In particular, the adaptive filter 108 digitally alters pulses in the signal 121 into a shape that is more suitable for processing by subsequent decoding circuitry (not shown). The adaptive filter 108 continually improves its performance by adjusting coefficients in response to an error signal 125. It should be appreciated that improving the performance of the adaptive filter 108 will reduce data errors in the signal 126.
The error signal circuit 110 receives a signal 123 that is a copy of the of the signal 122. An adder 112 in the error signal circuit 110 effectively subtracts the signal 123 from an ideal signal 124 to generate the error signal 125. The ideal signal 124 can be generated in numerous ways, such as using a slicer on the signal 123 or by using a digital copy of the data stored on the disk device 106. As mentioned above, the adaptive filter 108 uses the error signal 125 to modify its coefficients and improve bit error rate performance.
Unfortunately, the bit error rate performance of the conventional disk drive system 102 suffers because the adaptive filter coefficients do not converge to a solution for optimum bit error rate performance. The convergence problem is derived from the fact that the adaptive filter 108 has Least Mean Square (LMS) circuitry that adjusts the coefficients using a Mean Squared Error (MSE) driven process. Although MSE is a convenient metric that correlates with bit error rate, the correlation is not perfect. Thus, the convergence problem in the conventional disk drive 102 permits additional data errors to remain that prevent or slow the operation of the computer system 100. The additional data errors also require more expensive disk drive components to compensate for the errors.
Given the enormous growth in the demand for higher capacity computer data storage, there is an acute need to continually improve the performance of disk drive systems. In particular, solutions are needed to reduce the problem of data errors in disk drive systems. These solutions will allow less expensive components to be used while maintaining or improving current error rates. The cost savings can be passed on to the consumer in the form of less expensive computer data storage.